1. Field of the Invention
The present invention relates to a rudder angle sensor for detecting a rotation angle of a main drive gear (steering shaft) mounted in a vehicle such as an automobile.
2. Description of the Related Art
As a hitherto known rudder angle sensor (hereinafter referred to as “first conventional sensor” accordingly), there is one described in Patent Document 1 for example. The conventional rudder angle sensor is constituted by including a first gear interlocked with a main drive gear (steering shaft) to rotate and a second gear interlocked with the first gear to rotate at higher speed than the first gear. In the first conventional sensor, it is required to detect a rotation angle of the steering shaft with high accuracy, and as means to achieve this requirement, a third gear engaging with the first gear is provided therein.
[Patent Document 1] Japanese Patent Application Laid-open No. 2004-198287 (refer to paragraph 0016 and FIG. 3)
Further, a rudder angle sensor disclosed in Patent Document 2 (hereinafter referred to as “second conventional sensor” accordingly) is for detecting a front wheel rudder angle (rotation angle of the steering shaft) of a four-wheel steering vehicle, and determining a target rear wheel rudder angle for steering rear wheels according to the detected front wheel rudder angle. In order to prevent a minute variation in the detected front wheel rudder angle from taking effect, the second conventional sensor is constructed to perform sampling at predetermined time intervals and calculate the target rear wheel steering angle based on a weighted average value of the front wheel rudder angle which has been sampled from a predetermined time in advance to the current time. As a reason to adopt this structure, the Patent Document 2 describes that it is intended for preventing a minute variation from affecting the weighted average value when the minute variation occurs in the detected front wheel rudder angle.
[Patent Document 2] Japanese Patent Application Laid-open No. H07-25349 (refer to paragraph 0010 to 0020 and FIG. 3)
Furthermore, a rudder angle sensor disclosed in Patent Document 3 (hereinafter referred to as “third conventional sensor” accordingly) is constituted by including a hall element and a magnet. In the third conventional sensor, it is required to detect a rotation angle of the steering shaft with high accuracy, and as means to achieve this requirement, a ring-shaped ferromagnetic material is provided around the magnet. Since a detecting means is constituted of a plurality of magnetic detecting means, there is mutual magnetic interference between magnets. Patent Document 3 describes that this interference causes a problem of decreasing accuracy in detection values of the rotation angle, but since the third conventional sensor is provided with the ring-shaped ferromagnetic material around the magnet and a distance between the magnet and an ARM element is limited to approximately one-tenth or smaller as compared to the distance between the magnet and the ferromagnetic material, magnetic lines of force generated from two magnets are well prevented from leaking, and thus high accuracy in detection can be maintained without disturbing the magnetic fields of the magnets with each other. [Patent Document 3] Japanese Patent Application Laid-open No. 2004-271427 (refer to paragraph 0017, paragraph 0023, FIG. 1)
It is needless to mention that high accuracy is required for rudder angle sensors, but there are various factors to disturb the high accuracy. For example, according to a change in surrounding environment where an automobile is used, there may occur drifting in temperature, humidity or the like in electronic parts including the rudder angle sensor. In this case, it is possible that adverse effects due to the above drifting turn to abnormal data and eventually affect the accuracy in detection. With only the combination of gears adopted in the first conventional sensor, such abnormal data cannot be eliminated. Further, even with the rudder angle detection by means of weighted average value of sampled front wheel rudder angles adopted in the second conventional sensor, the above abnormal data cannot be eliminated either. This is because the second conventional sensor is constructed only for eliminating effects by a minute variation in the front wheel rudder angle. Moreover, with the third conventional sensor, it is not possible to completely eliminate effects of the magnet. The third conventional sensor asserts that there is no mutual interference of magnetism since, as described above, the distance between the magnet and the hall element (ARM element) is limited to approximately one-tenth or smaller as compared to the distance between the magnet and the ferromagnetic material (paragraph 0023), but this distance depends on a mutual relationship with the magnetic force of the magnet, and therefore it is hard to say that the decrease in accuracy of detection is completely prevented.
Accordingly, an object of the present invention is to provide a rudder angle sensor with higher accuracy by, firstly, preventing decrease in accuracy of detection of a rotation angle based on detection of abnormal data, and secondly, eliminating mutual interference of magnetism.